Manipulation of Molecules by Combined Permanent and Induced Dipole Forces

Abstract

This chapter describes a versatile technique to control molecular rotation and translation. The technique is based on the combined effect of external electric fields that act concurrently on the molecular permanent and induced dipole moments. This synergistic effect arises for any polar molecule, as only an anisotropic polarizability, along with a permanent dipole moment, is required. This is always available in polar molecules. The effect of the combined fields can be achieved either by superimposing an electrostatic and a far-off resonant optical field (i.e., one that supplies electric field strength but does not cause transitions) that act, respectively, on the molecule's permanent and induced electric dipole, or by making use of a unipolar electromagnetic pulse that seizes the permanent and induced dipole moments simultaneously. If the combined electric fields are homogeneous, only molecular rotation is affected, whereas inhomogeneous fields affect both molecular rotation and translation. As noted in the chapter, the ability to manipulate molecular rotation and translation has wide-ranging applications – in research areas as diverse as reaction dynamics, spectroscopy, higher harmonic generation and molecular orbital imaging, the focusing and trapping of molecules, as well as quantum simulation and computing.

My Vision of Electric-Field-Aided Chemistry in 2050

“…In summary, therefore, the application of OEEFs along the reaction axis will accelerate nonpolar reactions, control product selectivity by a flip of the field, control spin-state selectivity, , mechanistic selection, and so on... Furthermore, the application of an OEEF serves as tweezers that orient molecules in space − and make them react (along the reaction axis). OEEF orientations “off the reaction axis” were found to control selectivity patterns and chiral discrimination in Diels–Alder reactions .…”

“…It is often said that a major problem for implementing the usage of OEEFs is the requirement to orient the reactants along the OEEF. However, this problem does not really exist since the OEEF itself will orient the molecular complex in space , by increasing its dipole moment and optimizing the interaction energy with the OEEF. − Thus, for example, the interaction energy of the OEEF and the halogen bond between NH 3 and Cl 2 , in Scheme , is 25.3 kcal/mol . The formation of the halogen bond complex H 3 N–Cl–Cl is followed by a barrier of 21.8 kcal/mol for the Cl–Cl bond cleavage by the base.…”

An Electro-Optical Trap for Polar Molecules

“…where J 2 is the operator of the angular momentum squared and B is the rotational constant 21,22,25 . By dividing through B, the Hamiltonian becomes dimensionless,…”

“…3. The tunneling splitting scales as ∝ exp(−∆ζ 1/2 ) which means that the members of a given tunneling doublet can be drawn arbitrarily close to one another by boosting ∆ζ 25 . This is exemplified in the figure by the…”

“…In addition, an optical trap makes the trapped molecules directional -aligned -by virtue of the anisotropic interaction of the molecular polarizability with the polarization vector of the optical trapping field 21,22 . If the trapped molecules are polar, their alignment (which corresponds to a double-headed arrow) by the optical field can be converted to orientation (corresponding to a single-headed arrow) by superimposing an electrostatic field [23][24][25] . Polar molecules confined in tweezer arrays 26,27 entangled via the electric-dipole-dipole interaction between the molecules have been envisioned as platforms for quantum computing with the oriented molecular states serving as qubits 11,28 .…”